Vacancy, dominsated, defect-free silicon

ABSTRACT

The present invention relates to a process for growing a single crystal silicon. The process including controlling a growth velocity, v, and an average axial temperature gradient, G 0 , during the growth of the constant diameter portion of the crystal over the temperature range from solidification to a temperature of no less than about 1325° C., to cause the formation of a first axially symmetrical region in which vacancies, upon cooling of the ingot from the solidification temperature, are the predominant intrinsic point defect and which is substantially free of agglomerated intrinsic point defects, wherein the first axially symmetric region has a width of at least about 50% of the radius of the constant diameter portion of the ingot.

REFERENCE TO RELATED APPLICATION

This application is a U.S. divisional patent application which claimspriority from U.S. continuation-in-part application Ser. No. 09/270,366,filed on Mar. 16, 1999, now U.S. Pat. No. 6,379,642 which CIP of U.S.application Ser. No. 09/057,851, filed on Apr. 9, 1998, now U.S. Pat.No. 5,919,302 and U.S. provisional application Ser. No. 60/041,845,filed on Apr. 9, 1997.

BACKGROUND OF THE INVENTION

The present invention generally relates to the preparation ofsemiconductor grade single crystal silicon which is used in themanufacture of electronic components. More particularly, the presentinvention relates to single crystal silicon ingots and wafers having anaxially symmetric region of vacancy dominated material which is devoidof agglomerated intrinsic point defects, and a process for thepreparation thereof.

Single crystal silicon, which is the starting material for mostprocesses for the fabrication of semiconductor electronic components, iscommonly prepared by the so-called Czochralski (“Cz”) method. In thismethod, polycrystalline silicon (“polysilicon”) is charged to a crucibleand melted, a seed crystal is brought into contact with the moltensilicon and a single crystal is grown by slow extraction. Afterformation of a neck is complete, the diameter of the crystal is enlargedby decreasing the pulling rate and/or the melt temperature until thedesired or target diameter is reached. The cylindrical main body of thecrystal which has an approximately constant diameter is then grown bycontrolling the pull rate and the melt temperature while compensatingfor the decreasing melt level. Near the end of the growth process butbefore the crucible is emptied of molten silicon, the crystal diametermust be reduced gradually to form an end-cone. Typically, the end-coneis formed by increasing the crystal pull rate and heat supplied to thecrucible. When the diameter becomes small enough, the crystal is thenseparated from the melt.

In recent years, it has been recognized that a number of defects insingle crystal silicon form in the crystal growth chamber as the crystalcools after solidification. Such defects arise, in part, due to thepresence of an excess (i.e. a concentration above the solubility limit)of intrinsic point defects, which are known as vacancies andself-interstitials. Silicon crystals grown from a melt are typicallygrown with an excess of one or the other type of intrinsic point defect,either crystal lattice vacancies (“V”) or silicon self-interstitials(“I”). It has been suggested that the type and initial concentration ofthese point defects in the silicon are determined at the time ofsolidification and, if these concentrations reach a level of criticalsupersaturation in the system and the mobility of the point defects issufficiently high, a reaction, or an agglomeration event, will likelyoccur. Agglomerated intrinsic point defects in silicon can severelyimpact the yield potential of the material in the production of complexand highly integrated circuits.

Vacancy-type defects are recognized to be the origin of such observablecrystal defects as D-defects, Flow Pattern Defects (FPDs), Gate OxideIntegrity (GOI) Defects, Crystal Originated Particle (COP) Defects,crystal originated Light Point Defects (LPDs), as well as certainclasses of bulk defects observed by infrared light scattering techniquessuch as Scanning Infrared Microscopy and Laser Scanning Tomography. Alsopresent in regions of excess vacancies are defects which act as thenuclei for ring oxidation induced stacking faults (OISF). It isspeculated that this particular defect is a high temperature nucleatedoxygen agglomerate catalyzed by the presence of excess vacancies.

Defects relating to self-interstitials are less well studied. They aregenerally regarded as being low densities of interstitial-typedislocation loops or networks. Such defects are not responsible for gateoxide integrity failures, an important wafer performance criterion, butthey are widely recognized to be the cause of other types of devicefailures usually associated with current leakage problems.

The density of such vacancy and self-interstitial agglomerated defectsin Czochralski silicon is conventionally within the range of about1*10³/cm³ to about 1*10⁷/cm³. While these values are relatively low,agglomerated intrinsic point defects are of rapidly increasingimportance to device manufacturers and, in fact, are now seen asyield-limiting factors in device fabrication processes.

To date, there generally exists three main approaches to dealing withthe problem of-agglomerated intrinsic point defects. The first approachincludes methods which focus on crystal pulling techniques in order toreduce the number density of agglomerated intrinsic point defects in theingot. This approach can be further subdivided into those methods havingcrystal pulling conditions which result in the formation of vacancydominated material, and those methods having crystal pulling conditionswhich result in the formation of self-interstitial dominated material.For example, it has been suggested that the number density ofagglomerated defects can be reduced by (i) controlling v/G₀ to grow acrystal in which crystal lattice vacancies are the dominant intrinsicpoint defect, and (ii) influencing the nucleation rate of theagglomerated defects by altering (generally, by slowing down) thecooling rate of the silicon ingot from about 1100° C. to about 1050° C.during the crystal pulling process. While this approach reduces thenumber density of agglomerated defects, it does not prevent theirformation. As the requirements imposed by device manufacturers becomemore and more stringent, the presence of these defects will continue tobecome more of a problem.

Others have suggested reducing the pull rate, during the growth of thebody of the crystal, to a value less than about 0.4 mm/minute. Thissuggestion, however, is also not satisfactory because such a slow pullrate leads to reduced throughput for each crystal puller. Moreimportantly, such pull rates lead to the formation of single crystalsilicon having a high concentration of self-interstitials. This highconcentration, in turn, leads to the formation of agglomeratedself-interstitial defects and all the resulting problems associated withsuch defects.

A second approach to dealing with the problem of agglomerated intrinsicpoint defects includes methods which focus on the dissolution orannihilation of agglomerated intrinsic point defects subsequent to theirformation. Generally, this is achieved by using high temperature heattreatments of the silicon in wafer form. For example, Fusegawa et al.propose, in European Patent Application 503,816 A1, growing the siliconingot at a growth rate in excess of 0.8 mm/minute, and heat treating thewafers which are sliced from the ingot at a temperature in the range of1150° C. to 1280° C. to reduce the defect density in a thin region nearthe wafer surface. The specific treatment needed will vary dependingupon the concentration and location of agglomerated intrinsic pointdefects in the wafer. Different wafers cut from a crystal which does nothave a uniform axial concentration of such defects may require differentpost-growth processing conditions. Furthermore, such wafer heattreatments are relatively costly, have the potential for introducingmetallic impurities into the silicon wafers, and are not universallyeffective for all types of crystal-related defects.

A third approach to dealing with the problem of agglomerated intrinsicpoint defects is the epitaxial deposition of a thin crystalline layer ofsilicon on the surface of a single crystal silicon wafer. This processprovides a single crystal silicon wafer having a surface which issubstantially free of agglomerated intrinsic point defects. Epitaxialdeposition, however, substantially increases the cost of the wafer.

In view of these developments, a need continues to exist for a method ofsingle crystal silicon preparation which acts to prevent the formationof agglomerated intrinsic point defects by suppressing the-agglomerationreactions which produce them. Rather than simply limiting the rate atwhich such defects form, or attempting to annihilate some of the defectsafter they have formed, a method which acts to suppress agglomerationreactions would yield a silicon substrate that is substantially free ofagglomerated intrinsic point defects. Such a method would also affordsingle crystal silicon wafers having epi-like yield potential, in termsof the number of integrated circuits obtained per wafer, without havingthe high costs associated with an epitaxial process.

SUMMARY OF THE INVENTION

Among the objects of the present invention, therefore, is the provisionof single crystal silicon in ingot or wafer form having an axiallysymmetric region of substantial radial width which is substantially freeof defects resulting from an agglomeration of crystal lattice vacanciesor silicon self-interstitials; and the provision of a process forpreparing a single crystal silicon ingot in which the concentration ofvacancies and self-interstitials is controlled in order to prevent anagglomeration of intrinsic point defects in an axially symmetric segmentof a constant diameter portion of the ingot, as the ingot cools from thesolidification temperature.

Briefly, therefore, the present invention is directed to a singlecrystal silicon wafer having a central axis, a front side and a backside which are generally perpendicular to the central axis, acircumferential edge, and a radius extending from the central axis tothe circumferential edge of the wafer. The wafer comprises a firstaxially symmetric region in which vacancies are the predominantintrinsic point defect and which is substantially free of agglomeratedvacancy intrinsic point defects wherein the first axially symmetricregion has a width of at least about 60% of the radius of the wafer.

The present invention is further directed to a single crystal siliconingot having a central axis, a seed-cone, an end-cone, and a constantdiameter portion between the seed-cone and the end-cone having acircumferential edge and a radius extending from the central axis to thecircumferential edge. The single crystal silicon ingot is characterizedin that after the ingot is grown and cooled from the solidificationtemperature, the constant diameter portion contains a first axiallysymmetric region in which vacancies are the predominant intrinsic pointdefect and which is substantially free of agglomerated intrinsic pointdefects wherein the first axially symmetric region has a width of atleast about 50% of the radius of the ingot and has a length as measuredalong the central axis of at least about 20% of the length of theconstant diameter portion of the ingot.

The present invention is further directed to a process for growing asingle crystal silicon ingot in which the ingot comprises a centralaxis, a seed-cone, an end-cone and a constant diameter portion betweenthe seed-cone and the end-cone having a circumferential edge and aradius extending from the central axis to the circumferential edge. Inthis process, the ingot is grown from a silicon melt and then cooledfrom the solidification temperature in accordance with the Czochralskimethod. The process comprises controlling a growth velocity, v, and anaverage axial temperature gradient, G₀, during the growth of theconstant diameter portion of the crystal over the temperature range fromsolidification to a temperature of no less than about 1325° C., to causethe formation of a first axially symmetrical segment in which vacancies,upon cooling of the ingot from the solidification temperature, are thepredominant intrinsic point defect and which is substantially free ofagglomerated intrinsic point defects wherein the first axially symmetricregion extends has a width of at least about 50% of the radius of theingot.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which shows an example of how the initialconcentration of self-interstitials, [I], and vacancies, [V], changeswith an increase in the value of the ratio v/G₀, where v is the growthrate and G₀ is the average axial temperature gradient.

FIG. 2 is a graph which shows an example of how ΔG_(I), the change infree energy required for the formation of agglomerated interstitialdefects, increases as the temperature, T, decreases, for a given initialconcentration of self-interstitials, [I].

FIG. 3 is a graph which shows an example of how the initialconcentration of self-interstitials, [I], and vacancies, [V], can changealong the radius of an ingot or wafer, as the value of the ratio v/G₀decreases, due to an increase in the value of G₀. Note that at the V/Iboundary a transition occurs from vacancy dominated material toself-interstitial dominated material.

FIG. 4 is a top plan view of a single crystal silicon ingot or wafershowing regions of vacancy, V, and self-interstitial, I, dominatedmaterials respectively, as well as the V/I boundary that exists betweenthem.

FIG. 5 is a longitudinal, cross-sectional view of a single crystalsilicon ingot showing, in detail, an axially symmetric region of aconstant diameter portion of the ingot.

FIG. 6 is an image produced by a scan of the minority carrier lifetimeof an axial cut of the ingot following a series of oxygen precipitationheat treatments, showing in detail a generally cylindrical region ofvacancy dominated material, a generally annular shaped axially symmetricregion of self-interstitial dominated material, the V/I boundary presentbetween them, and a region of agglomerated interstitial defects.

FIG. 7 is a graph of pull rate (i.e. seed lift) as a function of crystallength, showing how the pull rate is decreased linearly over a portionof the length of the crystal.

FIG. 8 is an image produced by a scan of the minority carrier lifetimeof an axial cut of the ingot following a series of oxygen precipitationheat treatments, as described in Example 1.

FIG. 9 is a graph of pull rate as a function of crystal length for eachof four single crystal silicon ingots, labeled 1-4 respectively, whichare used to yield a curve, labeled v*(Z), as described in Example 1.

FIG. 10 is a graph of the average axial temperature gradient at themelt/solid interface, G₀, as a function of radial position, for twodifferent cases as described in Example 2.

FIG. 11 is a graph of the initial concentration of vacancies, [V], orself-interstitials, [I], as a function of radial position, for twodifferent cases as described Example 2.

FIG. 12 is a graph of temperature as a function of axial position,showing the axial temperature profile in ingots for two different casesas described in Example 3.

FIG. 13 is a graph of the self-interstitial concentrations resultingfrom the two cooling conditions illustrated in FIG. 12 and as more fullydescribed in Example 3.

FIG. 14 is an image produced by a scan of the minority carrier lifetimeof an axial cut of an entire ingot following a series of oxygenprecipitation heat treatments, as described in Example 4.

FIG. 15 is a graph illustrating the position of the V/I boundary as afunction of the length of the single crystal silicon ingot, as describedin Example 5.

FIG. 16 a is an image produced by a scan of the minority carrierlifetime of an axial cut of a segment of an ingot, ranging from about100 mm to about 250 mm from the shoulder of the ingot, following aseries of oxygen precipitation heat treatments, as described in Example6.

FIG. 16 b is an image produced by a scan of the minority carrierlifetime of an axial cut of a segment of an ingot, ranging from about250 mm to about 400 mm from the shoulder of the ingot, following aseries of oxygen precipitation heat treatments, as described in Example6.

FIG. 17 is a graph of the axial temperature gradient, G₀, at variousaxial positions for an ingot, as described in Example 7.

FIG. 18 is a graph of the radial variations in the average axialtemperature gradient, G₀, at various for an ingot, as described inExample 7.

FIG. 19 is a graph illustrating the relationship between the width ofthe axially symmetric region and the cooling rate, as described inExample 7.

FIG. 20 is a photograph of an axial cut of a segment of an ingot,ranging from about 235 mm to about 350 mm from the shoulder of theingot, following copper decoration and a defect-delineating etch,described in Example 7.

FIG. 21 is a photograph of an axial cut of a segment of an ingot,ranging from about 305 mm to about 460 mm from the shoulder of theingot, following copper decoration and a defect-delineating etch,described in Example 7.

FIG. 22 is a photograph of an axial cut of a segment of an ingot,ranging from about 140 mm to about 275 mm from the shoulder of theingot, following copper decoration and a defect-delineating etch,described in Example 7.

FIG. 23 is a photograph of an axial cut of a segment of an ingot,ranging from about 600 mm to about 730 mm from the shoulder of theingot, following copper decoration and a defect-delineating etch,described in Example 7.

FIG. 24 is a graph illustrating the radial variations in the averageaxial temperature gradient, G₀(r), which may occur in hot zones ofvarious configurations.

FIG. 25 is a graph illustrating the axial temperature profile for aningot in four different hot zone configurations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Based upon experimental evidence to date, it appears that the type andinitial concentration of intrinsic point defects is initially determinedas the ingot cools from the temperature of solidification (i.e., about1410° C.) to a temperature greater than 1300° C. (i.e., at least about1325° C., at least about 1350° C. or even at least about 1375° C.). Thatis, the type and initial concentration of these defects are controlledby the ratio v/G₀, where v is the growth velocity and G₀ is the averageaxial temperature gradient over this temperature range.

Referring to FIG. 1, for increasing values of v/G₀, a transition fromdecreasingly self-interstitial dominated growth to increasingly vacancydominated growth occurs near a critical value of v/G₀ which, based uponcurrently available information, appears to be about 2.1×10⁻⁵ cm²/sK,where G₀ is determined under conditions in which the axial temperaturegradient is constant within the temperature range defined-above. At thiscritical value, the concentrations of these intrinsic point defects areat equilibrium.

As the value of v/G₀ exceeds the critical value, the concentration ofvacancies increases. Likewise, as the value of v/G₀ falls below thecritical value, the concentration of self-interstitials increases. Ifthese concentrations reach a level of critical supersaturation in thesystem, and if the mobility of the point defects is sufficiently high, areaction, or an agglomeration event, will likely occur. Agglomeratedintrinsic point defects in silicon can severely impact the yieldpotential of the material in the production of complex and highlyintegrated circuits.

In accordance with the present invention, it has been discovered thatthe reactions in which vacancies within the silicon matrix react toproduce agglomerated vacancy defects and in which self-interstitialswithin the silicon matrix react to produce agglomerated interstitialdefects can be suppressed. Without being bound to any particular theory,it is believed that the concentration of vacancies andself-interstitials is controlled during the growth and cooling of thecrystal ingot in the process of the present invention, such that thechange in free energy of the system never exceeds a critical value atwhich the agglomeration reactions spontaneously occur to produceagglomerated vacancy or interstitial defects.

In general, the change in system free energy available to drive thereaction in which agglomerated vacancy defects are formed from vacancypoint defects or agglomerated interstitial defects are formed fromself-interstitial atoms in single crystal silicon is governed byEquation (1): $\begin{matrix}{{\Delta\quad G_{V/I}} = {{kT}\quad{\ln\left( \frac{\left\lbrack {V/I} \right\rbrack}{\left\lbrack {V/I} \right\rbrack^{eq}} \right)}}} & (1)\end{matrix}$wherein

-   -   ΔG_(V/I) is the change in free energy for the reaction which        forms agglomerated vacancy defects or the reaction which forms        the interstitial defects, as applicable,    -   k is the Boltzmann constant,    -   T is the temperature in K,    -   [V/I] is the concentration of vacancies or interstitials, as        applicable, at a point in space and time in the single crystal        silicon, and    -   [V/I]^(eq) is the equilibrium concentration of vacancies or        interstitials, as applicable, at the same point in space and        time at which [V/I] occurs and at the temperature, T.        According to this equation, for a given concentration of        vacancies, [V], a decrease in the temperature, T, generally        results in an increase in ΔG_(v) due to a sharp decrease in        [V]^(eq) with temperature. Similarly, for a given concentration        of interstitials, [I], a decrease in the temperature, T,        generally results in an increase in ΔG_(I) due to a sharp        decrease in [I]^(eq) with temperature.

FIG. 2 schematically illustrates the change in ΔG_(I) and theconcentration of silicon self-interstitials for an ingot which is cooledfrom the temperature of solidification without simultaneously employingsome means for suppression of the concentration of siliconself-interstitials. As the ingot cools, ΔG_(I) increases according toEquation (1), due to the increasing supersaturation of [I], and theenergy barrier for the formation of agglomerated interstitial defects isapproached. As cooling continues, this energy barrier is eventuallyexceeded, at which point a reaction occurs. This reaction results in theformation of agglomerated interstitial defects and the concomitantdecrease in ΔG_(I) as the supersaturated system is relaxed, i.e., as theconcentration of [I] decreases.

Similarly, as an ingot is cooled from the temperature of solidificationwithout simultaneously employing some means for suppression of theconcentration of vacancies, ΔG_(v) increases according to Equation (1),due to the increasing supersaturation of [V], and the energy barrier forthe formation of agglomerated vacancy defects is approached. As coolingcontinues, this energy barrier is eventually exceeded, at which point areaction occurs. This reaction results in the formation of agglomeratedvacancy defects and the concomitant decrease in ΔG_(V) as thesupersaturated system is relaxed.

The agglomeration of vacancies and interstitials can be avoided inregions of vacancy and interstitial dominated material, respectively, asthe ingot cools from the temperature of solidification by maintainingthe free energy of the vacancy system and the interstitial system atvalues which are less than those at which agglomeration reactions occur.In other words, the system can be controlled so as to never becomecritically supersaturated in vacancies or interstitials. This can beachieved by establishing initial concentrations of vacancies andinterstitials (controlled by v/G₀(r) as hereinafter defined) which aresufficiently low such that critical supersaturation is never achieved.In practice, however, such concentrations are difficult to achieveacross an entire crystal radius and, in general, therefore, criticalsupersaturation may be avoided by suppressing the initial vacancyconcentration and the initial interstitial concentration subsequent tocrystal solidification, i.e., subsequent to establishing the initialconcentration determined by v/G₀(r).

Surprisingly, it has been found that due to the relatively largemobility of self-interstitials, which is generally about 10⁻⁴cm²/second, and to a lesser extent, to the mobility of vacancies, it ispossible to effect the suppression of interstitials and vacancies overrelatively large distances, i.e. distances of about 5 cm to about 10 cmor more, by the radial diffusion of self-interstitials to sinks locatedat the crystal surface or to vacancy dominated regions located withinthe crystal. Radial diffusion can be effectively used to suppress theconcentration of self-interstitials and vacancies, provided sufficienttime is allowed for the radial diffusion of the initial concentration ofintrinsic point defects. In general, the diffusion time will depend uponthe radial variation in the initial concentration of self-interstitialsand vacancies, with lesser radial variations requiring shorter diffusiontimes.

Typically, the average axial temperature gradient, G₀, increases as afunction of increasing radius for single crystal silicon, which is grownaccording to the Czochralski method. This means that the value of v/G₀is typically not singular across the radius of an ingot. As a result ofthis variation, the type and initial concentration of intrinsic pointdefects is not constant. If the critical value of v/G₀, denoted in FIGS.3 and 4 as the V/I boundary 2, is reached at some point along the radius4 of the ingot, the material will switch from being vacancy dominated toself-interstitial dominated. In addition, the ingot will contain anaxially symmetric region of self-interstitial dominated material 6 (inwhich the initial concentration of silicon self-interstitial atomsincreases as a function of increasing radius), surrounding a generallycylindrical region of vacancy dominated material 8 (in which the initialconcentration of vacancies decreases as a function of increasingradius).

As an ingot containing a V/I boundary is cooled from the temperature ofsolidification, radial diffusion of interstitial atoms and vacanciescauses a radially inward shift in the V/I boundary due to arecombination of self-interstitials with vacancies. In addition, radialdiffusion of self-interstitials to the surface of the crystal will occuras the crystal cools. The surface of the crystal is capable ofmaintaining near equilibrium point defect concentrations as the crystalcools. Radial diffusion of point defects will tend to reduce theself-interstitial concentration outside the V/I boundary and the vacancyconcentration inside the V/I boundary. If enough time is allowed fordiffusion, therefore, the concentration of vacancy and interstitialseverywhere may be such that ΔG_(V) and ΔG_(I) will be less than thecritical values at which the vacancy agglomeration reaction and theinterstitial agglomeration reactions occur.

Referring now to FIG. 5, a single crystal silicon ingot 10 is grown inaccordance with the Czochralski method in a first embodiment of theprocess of the present invention. The silicon ingot comprises a centralaxis 12, a seed-cone 14, an end-cone 16 and a constant diameter portion18 between the seed-cone and the end-cone. The constant diameter portionhas a circumferential edge 20 and a radius 4 extending from the centralaxis 12 to the circumferential edge 20.

The crystal growth conditions, including growth velocity, v, the averageaxial temperature gradient, G₀, and the cooling rate are preferablycontrolled to cause the formation of an axially symmetric region ofinterstitial dominated material 6, and a generally cylindrical region ofvacancy dominated material 8 which contains an axially symmetric regionof agglomerated intrinsic point defect-free material 9. Axiallysymmetric region 9 has a width, as measured along the radius extendingfrom V/I boundary 2 to axis 12, which in one embodiment of the presentinvention is at least about 15 mm in width, and preferably has a widthwhich is at least about 7.5%, more preferably at least about 15%, 25%,50%, 60% and even 80%, and most preferably at least about 90% of theradius of the constant diameter portion of the ingot. In a particularlypreferred embodiment, the axially symmetric region 9 includes axis 12 ofthe ingot, i.e., the axially symmetric region 9 and generallycylindrical region 8 coincide. Stated another way, ingot 10 includes agenerally cylindrical region of vacancy dominated material 8, at least aportion of which is free of agglomerated defects. In addition, axiallysymmetric region 9 extends over a length of at least about 20%,preferably at least about 40%, more preferably at least about 60%, stillmore preferably at least about 80%, and most preferably at least about90% of the length of the constant diameter portion of the ingot.

Axially symmetric region 6 (when present) generally has a width, asmeasured from circumferential edge 20 radially inward toward centralaxis 12, of at least about 30%, and in some embodiments at least about40%, at least about 60%, or even at least about 80% of the radius of theconstant diameter portion of the ingot. In addition, the axiallysymmetric region generally extends over a length of at least about 20%,preferably at least about 40%, more preferably at least about 60%, andstill more preferably at least about 80% of the length of the constantdiameter portion of the ingot.

The width of axially symmetric regions 6 and 9 may have some variationalong the length of the central axis 12. For an axially symmetric regionof a given length, therefore, the width of axially symmetric region 6 isdetermined by measuring the distance from the circumferential edge 20 ofthe ingot 10 radially toward a point which is farthest from the centralaxis. In other words, the width is measured such that the minimumdistance within the given length of the axially symmetric region 6 isdetermined. Similarly, the width of axially symmetric region 9 isdetermined by measuring the distance from the V/I boundary 2 radiallytoward a point which is farthest from the central axis. In other words,the width is measured such that the minimum distance within the givenlength of the axially symmetric region 9 is determined.

The growth velocity, v, and the average axial temperature gradient, G₀,(as previously defined) are typically controlled such that the ratiov/G₀ ranges in value from about 0.5 to about 2.5 times the criticalvalue of v/G₀ (i.e., about 1×10⁻⁵ cm²/sK to about 5×10⁻⁵ cm²/sK basedupon currently available information for the critical value of v/G₀).Preferably, the ratio v/G₀ will range in value from about 0.6 to about1.5 times the critical value of v/G₀ (i.e., about 1.3×10⁻⁵ cm²/sK toabout 3×10⁻⁵ cm²/sK based upon currently available information for thecritical value of v/G₀). Most preferably, the ratio v/G₀ will range invalue from about 0.75 to about 1.25 times the critical value of v/G₀(i.e., about 1.6×10⁻⁵ cm²/sK to about 2.1×10³¹ ⁵ cm²/sK based uponcurrently available information for the critical value of v/G₀). In aparticularly preferred embodiment, v/G₀ within the generally cylindricalregion 8 has a value falling between the critical value of v/G₀ and 1.1times the critical value of v/G₀.

To maximize the width of the axially symmetric region 9 it is preferredthat the ingot be cooled from the solidification temperature to atemperature in excess of about 1050° C. over a period of (i) at leastabout 5 hours, preferably at least about 10 hours, and more preferablyat least about 15 hours for 150 mm nominal diameter silicon crystals,(ii) at least about 5 hours, preferably at least about 10 hours, morepreferably at least about 20 hours, still more preferably at least about25 hours, and most preferably at least about 30 hours for 200 mm nominaldiameter silicon crystals, and (iii) at least about 20 hours, preferablyat least about 40 hours, more preferably at least about 60 hours, andmost preferably at least about 75 hours for silicon crystals having anominal diameter greater than 200 mm. Control of the cooling rate can beachieved by using any means currently known in the art for minimizingheat transfer, including the use of insulators, heaters, radiationshields, and magnetic fields.

Control of the average axial temperature gradient, G₀, may be achievedthrough the design of the “hot zone” of the crystal puller, i.e. thegraphite (or other materials) that makes up the heater, insulation, heatand radiation shields, among other things. Although the designparticulars may vary depending upon the make and model of the crystalpuller, in general, G₀ may be controlled using any of the meanscurrently known in the art for controlling heat transfer at themelt/solid interface, including reflectors, radiation shields, purgetubes, light pipes, and heaters. In general, radial variations in G₀ areminimized by positioning such an apparatus within about one crystaldiameter above the melt/solid interface. G₀ can be controlled further byadjusting the position of the apparatus relative to the melt andcrystal. This is accomplished either by adjusting the position of theapparatus in the hot zone, or by adjusting the position of the meltsurface in the hot zone. In addition, when a heater is employed, G₀ maybe further controlled by adjusting the power supplied to the heater.Any, or all, of these methods can be used during a batch Czochralskiprocess in which melt volume is depleted during the process.

It is generally preferred for some embodiments of the present inventionthat the average axial temperature gradient, G₀, be relatively constantas a function of diameter of the ingot. However, it should be noted thatas improvements in hot zone design allow for variations in G₀ to beminimized, mechanical issues associated with maintaining a constantgrowth rate become an increasingly important factor. This is because thegrowth process becomes much more sensitive to any variation in the pullrate, which in turn directly effects the growth rate, v. In terms ofprocess control, this means that it is favorable to have values for G₀which differ over the radius of the ingot. Significant differences inthe value of G₀, however, can result in a large concentration ofself-interstitials generally increasing toward the wafer edge and,thereby, increase the difficultly in avoiding the formation ofagglomerated intrinsic point defects.

In view of the foregoing, the control of G₀ involves a balance betweenminimizing radial variations in G₀ and maintaining favorable processcontrol conditions. Typically, therefore, the pull rate after about onediameter of the crystal length will range from about 0.2 mm/minute toabout 0.8 mm/minute. Preferably, the pull rate will range from about0.25 mm/minute to about 0.6 mm/minute and, more preferably, from about0.3 mm/minute to about 0.5 mm/minute. It is to be noted that the pullrate is dependent upon both the crystal diameter and crystal pullerdesign. The stated ranges are typical for 200 mm diameter crystals. Ingeneral, the pull rate will decrease as the crystal diameter increases.However, the crystal puller may be designed to allow pull rates inexcess of those stated here. As a result, most preferably the crystalpuller will be designed to enable the pull rate to be as fast aspossible while still allowing for the formation of an axially symmetricregion in accordance with the present invention.

The amount of self-interstitial diffusion is controlled by controllingthe cooling rate as the ingot is cooled from the solidificationtemperature (about 1410° C.) to the temperature at which siliconself-interstitials become immobile, for commercially practical purposes.Silicon self-interstitials appear to be extremely mobile at temperaturesnear the solidification temperature of silicon, i.e. about 1410° C. Thismobility, however, decreases as the temperature of the single crystalsilicon ingot decreases. Generally, the diffusion rate ofself-interstitials slows such a considerable degree that they areessentially immobile for commercially practical time periods attemperatures less than about 700° C., and perhaps at temperatures asgreat as 800° C., 900° C., 1000° C., or even 1050° C.

It is to be noted in this regard that, although the temperature at whicha self-interstitial agglomeration reaction occurs may in theory varyover a wide range of temperatures, as a practical matter this rangeappears to be relatively narrow for conventional, Czochralski grownsilicon. This is a consequence of the relatively narrow range of initialself-interstitial concentrations which are typically obtained in silicongrown according to the Czochralski method. In general, therefore, aself-interstitial agglomeration reaction may occur, if at all, attemperatures within the range of about 1100° C. to about 800° C., andtypically at a temperature of about 1050° C.

Within the range of temperatures at which self-interstitials appear tobe mobile, and depending upon the temperature in the hot zone, thecooling rate will typically range from about 0.1° C./minute to about 3°C./minute. Preferably, the cooling rate will range from about 0.1°C./minute to about 1.5° C./minute, more preferably from about 0.1°C./minute to about 1° C./minute, and still more preferably from about0.1° C./minute to about 0.5° C./minute.

By controlling the cooling rate of the ingot within a range oftemperatures in which self-interstitials appear to be mobile, theself-interstitials may be given more time to diffuse to sinks located atthe crystal surface, or to vacancy dominated regions, where they may beannihilated. The concentration of such interstitials may therefore besuppressed, which act to prevent an agglomeration event from occurring.Utilizing the diffusivity of interstitials by controlling the coolingrate acts to relax the otherwise stringent v/G₀ requirements that may berequired in order to obtain an axially symmetric region free ofagglomerated defects. Stated another way, as a result of the fact thatthe cooling rate may be controlled in order to allow interstitials moretime to diffuse, a large range of v/G₀ values, relative to the criticalvalue, are acceptable for purposes of obtaining an axially symmetricregion free of agglomerated defects.

To achieve such cooling rates over appreciable lengths of the constantdiameter portion of the crystal, consideration must also be given to thegrowth process of the end-cone of the ingot, as well as the treatment ofthe ingot once end-cone growth is complete. Typically, upon completionof the growth of the constant diameter portion of the ingot, the pullrate will be increased in order to begin the tapering necessary to formthe end-cone. However, such an increase in pull rate will result in thelower segment of the constant diameter portion cooling more quicklywithin the temperature range in which interstitials are sufficientlymobile, as discussed above. As a result, these interstitials may nothave sufficient time to diffuse to sinks to be annihilated; that is, theconcentration in this lower segment may not be suppressed to asufficient degree and agglomeration of interstitial defects may result.

In order to prevent the formation of such defects from occurring in thislower segment of the ingot, it is therefore preferred that constantdiameter portion of the ingot have a uniform thermal history inaccordance with the Czochralski method. A uniform thermal history may beachieved by pulling the ingot from the silicon melt at a relativelyconstant rate during the growth of not only the constant diameterportion, but also during the growth of the end-cone of the crystal andpossibly subsequent to growth of the end-cone. The relatively constantrate may be achieved, for example, by (i) reducing the rates of rotationof the crucible and crystal during the growth of the end-cone relativeto the crucible and crystal rotation rates during the growth of theconstant diameter portion of the crystal, and/or (ii) increasing thepower supplied to the heater used to heat the silicon melt during thegrowth of the end-cone relative to the power conventionally suppliedduring end-cone growth. These additional adjustments of the processvariables may occur either individually or in combination.

When the growth of the end-cone is initiated, a pull rate for theend-cone is established such that, any segment of the constant diameterportion of the ingot which remains at a temperature in excess of about1050° C. experiences the same thermal history as other segment(s) of theconstant diameter portion of the ingot which contain an axiallysymmetric region free of agglomerated intrinsic point defects which havealready cooled to a temperature of less than about 1050° C.

As previously noted, a minimum radius of the vacancy dominated regionexists for which the suppression of agglomerated interstitial defectsmay be achieved. The value of the minimum radius depends on v/G₀(r) andthe cooling rate. As crystal puller and hot zone designs will vary, theranges presented above for v/G₀(r), pull rate, and cooling rate willalso vary. Likewise these conditions may vary along the length of agrowing crystal. Also as noted above, the width of the interstitialdominated region free of agglomerated interstitial defects is preferablymaximized. Thus, it is desirable to maintain the width of this region toa value which is as close as possible to, without exceeding, thedifference between the crystal radius and the minimum radius of thevacancy dominated region along the length of the growing crystal in agiven crystal puller.

The optimum width of axially symmetric regions 6 and 9 and the requiredoptimal crystal pulling rate profile for a given crystal puller hot zonedesign may be determined empirically. Generally speaking, this empiricalapproach involves first obtaining readily available data on the axialtemperature profile for an ingot grown in a particular crystal puller,as well as the radial variations in the average axial temperaturegradient for an ingot grown in the same puller. Collectively, this datais used to pull one or more single crystal silicon ingots, which arethen analyzed for the presence of agglomerated interstitial defects. Inthis way, an optimum pull rate profile can be determined.

FIG. 6 is an image produced by a scan of the minority carrier lifetimeof an axial cut of a section of a 200 mm diameter ingot following aseries of oxygen precipitation heat-treatments which reveal defectdistribution patterns. It depicts an example in which a near-optimumpull rate profile is employed for a given crystal puller hot zonedesign. In this example, a transition occurs from a v/G₀(r) at which themaximum width of the interstitial dominated region is exceeded(resulting in the generation of regions of agglomerated interstitialdefects 28) to an optimum v/G₀(r) at which the axially symmetric regionhas the maximum width.

In addition to the radial variations in v/G₀ resulting from an increasein G₀ over the radius of the ingot, v/G₀ may also vary axially as aresult of a change in v, or as a result of natural variations in G₀ dueto the Czochralski process. For a standard Czochralski process, v isaltered as the pull rate is adjusted throughout the growth cycle, inorder to maintain the ingot at a constant diameter. These adjustments,or changes, in the pull rate in turn cause v/G₀ to vary over the lengthof the constant diameter portion of the ingot. In accordance with theprocess of the present invention, the pull rate is therefore controlledin order to maximize the width of the axially symmetric region of theingot. As a result, however, variations in the radius of the ingot mayoccur. In order to ensure that the resulting ingot has a constantdiameter, the ingot is therefore preferably grown to a diameter largerthan that which is desired. The ingot is then subjected to processesstandard in the art to remove excess material from the surface, thusensuring that an ingot having a constant diameter portion is obtained.

In general, it is easier to make vacancy dominated material free ofagglomerated defects when radial variation of the axial temperaturegradient, G₀(r), is minimized. Referring to FIG. 25, axial temperatureprofile for four separate hot zone configurations are illustrated. FIG.24 presents the variation in the axial temperature gradient, G₀(r), fromthe center of the crystal to one-half of the crystal radius, determinedby averaging the gradient from the solidification temperature to thetemperature indicated on the x-axis. When crystals were pulled in hotzones designated as Ver. 1 and Ver. 4 which have larger radial variationin G₀(r), it was not possible to obtain crystals having vacancydominated material from center to edge of any axial length which wasfree of agglomerated defects. When crystals were pulled in hot zonesdesignated as Ver. 2 and Ver. 3 which have lesser radial variation inG₀(r), however, it was possible to obtain crystals having vacancydominated material from center to edge which was free of agglomerateddefects for some axial length of the crystal.

For an ingot prepared in accordance with the process of the presentinvention and having a V/I boundary, i.e. an ingot containing materialwhich is vacancy dominated, experience has shown that low material,i.e., less than about 13 PPMA (parts per million atomic, ASTM standardF-121-83), is preferred. More preferably, the single crystal siliconcontains less than about 12 PPMA oxygen, still more preferably less thanabout 11 PPMA oxygen, and most preferably less than about 10 PPMAoxygen. This is because, in medium to high oxygen contents wafers, i.e.,14 PPMA to 18 PPMA, the formation of oxygen-induced stacking faults andbands of enhanced oxygen clustering just inside the V/I boundary becomesmore pronounced. Each of these are a potential source for problems in agiven integrated circuit fabrication process.

The effects of enhanced oxygen clustering may be further reduced by anumber of methods, used singularly or in combination. For example,oxygen precipitate nucleation centers typically form in silicon which isannealed at a temperature in the range of about 350° C. to about 750° C.For some applications, therefore, it may be preferred that the crystalbe a “short” crystal, that is, a crystal which has been grown in aCzochralski process until the seed end has cooled from the melting pointof silicon (about 1410° C.) to about 750° C. after which the ingot israpidly cooled. In this way, the time spent in the temperature rangecritical for nucleation center formation is kept to a minimum and theoxygen precipitate nucleation centers have inadequate time to form inthe crystal puller.

Preferably, however, oxygen precipitate nucleation centers formed duringthe growth of the single crystal are dissolved by annealing the singlecrystal silicon. Provided they have not been subjected to a stabilizingheat-treatment, oxygen precipitate nucleation centers can be annealedout of silicon by rapidly heating the silicon to a temperature of atleast about 875° C., and preferably continuing to increase thetemperature to at least 1000° C., at least 1100° C., or more. By thetime the silicon reaches 1000° C., substantially all (e.g.,>99%) of suchdefects have annealed out. It is important that the wafers be rapidlyheated to these temperatures, i.e., that the rate of temperatureincrease be at least about 10° C. per minute and more preferably atleast about 50° C. per minute. Otherwise, some or all of the oxygenprecipitate nucleation centers may be stabilized by the heat-treatment.Equilibrium appears to be reached in relatively short periods of time,i.e., on the order of about 60 seconds or less. Accordingly, oxygenprecipitate nucleation centers in the single crystal silicon may bedissolved by annealing it at a temperature of at least about 875° C.,preferably at least about 95.0° C., and more preferably at least about1100° C., for a period of at least about 5 seconds, and preferably atleast about 10 minutes.

The dissolution may be carried out in a conventional furnace or in arapid thermal annealing (RTA) system. The rapid thermal anneal ofsilicon may be carried out in any of a number of commercially availablerapid thermal annealing (“RTA”) furnaces in which wafers areindividually heated by banks of high power lamps. RTA furnaces arecapable of rapidly heating a silicon wafer, e.g., they are capable ofheating a wafer from room temperature to 1200° C. in a few seconds. Onesuch commercially available RTA furnace is the model 610 furnaceavailable from AG Associates (Mountain View, Calif.). In addition, thedissolution may be carried out on silicon ingots or on silicon wafers,preferably wafers.

In one embodiment of the process of the present invention, the initialconcentration of silicon self-interstitial atoms is controlled in theaxially symmetric, self-interstitial dominated region 6 of ingot 10.Referring again to FIG. 1, in general, the initial concentration ofsilicon self-interstitial atoms is controlled by controlling the crystalgrowth velocity, v, and the average axial temperature gradient, G₀, suchthat the value of the ratio v/G₀ is relatively near the critical valueof this ratio, at which the V/I boundary occurs. In addition, theaverage axial temperature gradient, G₀, can be established such that thevariation of G₀, i.e. G₀(r), (and thus, v/G₀(r)) as a the variation ofG₀ (and thus, v/G₀) as a function of the ingot radius is alsocontrolled.

In another embodiment of the present invention, V/G₀ is controlled suchthat no V/I boundary exists along the radius for at least a portion ofthe length of the ingot. In this length, the silicon is vacancydominated from center to circumferential edge and agglomerated vacancydefects are avoided in an axially symmetric region extending radiallyinward from the circumferential edge of the ingot principally bycontrolling V/G₀. That is, the growth conditions are controlled so thatv/G₀ has a value falling between the critical value of v/G₀ and 1.1times the critical value of v/G₀.

It is to be noted that wafers prepared in accordance with the presentinvention are suitable for use as substrates upon which an epitaxiallayer may be deposited. Epitaxial deposition may be performed by meanscommon in the art.

Furthermore, it is also to be noted that wafers prepared in accordancewith the present invention are suitable for use in combination withhydrogen or argon annealing treatments, such as the treatments describedin European Patent Application No. 503,816 A1.

Visual Detection of Agglomerated Defects

Agglomerated defects may be detected by a number of differenttechniques. For example, flow pattern defects, or D-defects, aretypically detected by preferentially etching the single crystal siliconsample in a Secco etch solution for about 30 minutes, and thensubjecting the sample to microscopic inspection. (see, e.g., H.Yamagishi et al., Semicond. Sci. Technol. 7, A135 (1992)). Althoughstandard for the detection of agglomerated vacancy defects, this processmay also be used to detect agglomerated interstitial defects. When thistechnique is used, such defects appear as large pits on the surface ofthe sample when present.

Agglomerated defects may also be detected using laser scatteringtechniques, such as laser scattering tomography, which typically have alower defect density detection limit that other etching techniques.

Additionally, agglomerated intrinsic point defects may be visuallydetected by decorating these defects with a metal capable of diffusinginto the single crystal silicon matrix upon the application of heat.Specifically, single crystal silicon samples, such as wafers, slugs orslabs, may be visually inspected for the presence of such defects byfirst coating a surface of the sample with a composition containing ametal capable of decorating these defects, such as a concentratedsolution of copper nitrate. The coated sample is then heated to atemperature between about 900° C. and about 1000° C. for about 5 minutesto about 15 minutes in order to diffuse the metal into the sample. Theheat treated sample is then cooled to room temperature, thus causing themetal to become critically supersaturated and precipitate at siteswithin the sample matrix at which defects are present.

After cooling, the sample is first subjected to a non-defect delineatingetch, in order to remove surface residue and precipitants, by treatingthe sample with a bright etch solution for about 8 to about 12 minutes.A typical bright etch solution comprises about 55 percent nitric acid(70% solution by weight), about 20 percent hydrofluoric acid (49%solution by weight), and about 25 percent hydrochloric acid(concentrated solution).

The sample is then rinsed with deionized water and subjected to a secondetching step by immersing the sample in, or treating it with, a Secco orWright etch solution for about 35 to about 55 minutes. Typically, thesample will be etched using a Secco etch solution comprising about a 1:2ratio of 0.15 M potassium dichromate and hydrofluoric acid (49% solutionby weight). This etching step acts to reveal, or delineate, agglomerateddefects which may be present.

In general, regions of interstitial and vacancy dominated material freeof agglomerated defects can be distinguished from each other and frommaterial containing agglomerated defects by the copper decorationtechnique described above. Regions of defect-free interstitial dominatedmaterial contain no decorated features revealed by the etching whereasregions of defect-free vacancy dominated material (prior to ahigh-temperature oxygen nuclei dissolution treatment as described above)contain small etch pits due to copper decoration of the oxygen nuclei.

Definitions

As used herein, the following phrases or terms shall have the givenmeanings: “agglomerated intrinsic point defects” mean defects caused (i)by the reaction in which vacancies agglomerate to produce D-defects,flow pattern defects, gate oxide integrity defects, crystal originatedparticle defects, crystal originated light point defects, and other suchvacancy related defects, or (ii) by the reaction in whichself-interstitials agglomerate to produce dislocation loops andnetworks, and other such self-interstitial related defects;“agglomerated interstitial defects” shall mean agglomerated intrinsicpoint defects caused by the reaction in which silicon self-interstitialatoms agglomerate; “agglomerated vacancy defects” shall meanagglomerated vacancy point defects caused by the reaction in whichcrystal lattice vacancies agglomerate; “radius” means the distancemeasured from a central axis to a circumferential edge of a wafer oringot; “substantially free of agglomerated intrinsic point defects”shall mean a concentration of agglomerated defects which is less thanthe detection limit of these defects, which is currently about 10³defects/cm³; “V/I boundary” means the position along the radius of aningot or wafer at which the material changes from vacancy dominated toself-interstitial dominated; and “vacancy dominated” and“self-interstitial dominated” mean material in which the intrinsic pointdefects are predominantly vacancies or self-interstitials, respectively.

EXAMPLES

As the following examples illustrate, the present invention affords aprocess for preparing a single crystal silicon ingot in which, as theingot cools from the solidification temperature in accordance with theCzochralski method, the agglomeration of intrinsic point defects isprevented within an axially symmetric region of the constant diameterportion of the ingot, from which wafers may be sliced.

The following examples set forth one set of conditions that may be usedto achieve the desired result. Alternative approaches exist fordetermining an optimum pull rate profile for a given crystal puller. Forexample, rather than growing a series of ingots at various pull rates, asingle crystal could be grown at pull rates which increase and decreasealong the length of the crystal; in this approach, agglomeratedself-interstitial defects would be caused to appear and disappearmultiple times during growth of a single crystal. Optimal pull ratescould then be determined for a number of different crystal positions.Accordingly, the following examples should not be interpreted in alimiting sense.

Example 1 Optimization Procedure for a Crystal Puller Having aPre-existing Hot Zone Design

A first 200 mm single crystal silicon ingot was grown under conditionsin which the pull rate was ramped linearly from about 0.75 mm/min. toabout 0.35 mm/min. over the length of the crystal. FIG. 7 shows the pullrate as a function of crystal length. Taking into account thepre-established axial temperature profile of a growing 200 mm ingot inthe-crystal-puller and the pre-established radial variations in theaverage axial temperature gradient, G₀, i.e., the axial temperaturegradient at the melt/solid interface, these pull rates were selected toinsure that ingot would be vacancy dominated material from the center tothe edge at one end of the ingot and interstitial dominated materialfrom the center to the edge of the other end of the ingot. The growningot was sliced longitudinally and analyzed to determine where theformation of agglomerated interstitial defects begins.

FIG. 8 is an image produced by a scan of the minority carrier lifetimeof an axial cut of the ingot over a section ranging from about 635 mm toabout 760 mm from the shoulder of the ingot following a series of oxygenprecipitation heat-treatments which reveal defect distribution patterns.At a crystal position of about 680 mm, a band of agglomeratedinterstitial defects 28 can be seen. This position corresponds to acritical pull rate of v*(680 mm)=0.33 mm/min. At this point, the widthof the axially symmetric region 6 (a region which is interstitialdominated material but which lacks agglomerated interstitial defects) isat its maximum; the width of the vacancy dominated region 8, R_(v)*(680)is about 35 mm and the width of the axially symmetric region,R_(I)*(680) is about 65 mm.

A series of four single crystal silicon ingots were then grown at steadystate pull rates which were somewhat greater than and somewhat less thanthe pull rate at which the maximum width of the axially symmetric regionof the first 200 mm ingot was obtained. FIG. 9 shows the pull rate as afunction of crystal length for each of the four crystals, labeled,respectively; as 1-4. These four crystals were then analyzed todetermine the axial position (and corresponding pull rate) at whichagglomerated interstitial defects first appear or disappear. These fourempirically determined points (marked “*”) are shown in FIG. 9.Interpolation between and extrapolation from these points yielded acurve, labeled v*(Z) in FIG. 9. This curve represents, to a firstapproximation, the pull rate for 200 mm crystals as a function of lengthin the crystal puller at which the axially symmetric region is at itsmaximum width.

Growth of additional crystals at other pull rates and further analysisof these crystals would further refine the empirical definition ofv*(Z).

Example 2 Reduction of Radial Variation in G₀(r)

FIGS. 10 and 11 illustrate the improvement in quality that can beachieved by reduction of the radial variation in the axial temperaturegradient at the melt/solid interface, G₀(r). The initial concentration(about 1 cm from the melt/solid interface) of vacancies andinterstitials are calculated for two cases with different G₀(r): (1)G₀(r)=2.65+5×10⁻⁴r² (K/mm) and (2) G₀(r)=2.65+5×10⁻⁵r² (K/mm). For eachcase the pull rate was adjusted such that the boundary betweenvacancy-rich silicon and interstitial-rich silicon is at a radius of 3cm. The pull rate used for case 1 and 2 were 0.4 and 0.35 mm/min,respectively. From FIG. 11 it is clear that the initial concentration ofinterstitials in the interstitial-rich portion of the crystal isdramatically reduced as the radial variation in the initial axialtemperature gradient is reduced. This leads to an improvement in thequality of the material since it becomes easier to avoid the formationof interstitial defect clusters due to supersaturation of interstitials.

Example 3 Increased Out-diffusion Time for Interstitials

FIGS. 12 and 13 illustrate the improvement in quality that can beachieved by increasing the time for out-diffusion of interstitials Theconcentration of interstitials is calculated for two cases withdiffering axial temperature profiles in the crystal, dT/dz. The axialtemperature gradient at the melt/solid interface is the same for bothcases, so that the initial concentration (about 1 cm from the melt/solidinterface) of interstitials is the same for both cases. In this example,the pull rate was adjusted such that the entire crystal isinterstitial-rich. The pull rate was the same for both cases, 0.32mm/min. The longer time for interstitial out-diffusion in case 2 resultsin an overall reduction of the interstitial concentration. This leads toan improvement in the quality of the material since it becomes easier toavoid the formation of interstitial defect clusters due tosupersaturation of interstitials.

Example 4

A 700 mm long, 150 mm diameter crystal was grown with a varying pullrate. The pull rate varied nearly linearly from about 1.2 mm/min at theshoulder to about 0.4 mm/min at 430 mm from the shoulder, and thennearly linearly back to about 0.65 mm/min at 700 mm from the shoulder.Under these conditions in this particular crystal puller, the entireradius is grown under interstitial-rich conditions over the length ofcrystal ranging from about 320 mm to about 525 mm from the shoulder ofthe crystal. Referring to FIG. 14, at an axial position of about 525 mmand a pull rate of about 0.47 mm/min, the crystal is free ofagglomerated intrinsic point defects clusters across the entirediameter. Stated another way, there is one small section of the crystalin which the width of the axially symmetric region, i.e., the regionwhich is substantially free of agglomerated defects, is equal to theradius of the ingot.

Example 5

As described in Example 1, a series of single crystal silicon ingotswere grown at varying pull rates and then analyzed to determine theaxial position (and corresponding pull rate) at which agglomeratedinterstitial defects first appeared or disappeared. Interpolationbetween and extrapolation from these points, plotted on a graph of pullrate v. axial position, yielded a curve which represents, to a firstapproximation, the pull rate for a 200 mm crystal as a function oflength in the crystal puller at which the axially symmetric region is atits maximum width. Additional crystals were then grown at other pullrates and further analysis of these crystals was used to refine thisempirically determined optimum pull rate profile.

Using this data and following this optimum pull rate profile, a crystalof about 1000 mm in length and about 200 mm in diameter was grown.Slices of the grown crystal, obtained from various axial position, werethen analyzed using oxygen precipitation methods standard in the art inorder to (i) determine if agglomerated interstitial defects were formed,and (ii) determine, as a function of the radius of the slice, theposition of the V/I boundary. In this way the presence of an axiallysymmetric region was determined, as well as the width of this region afunction of crystal length or position.

The results obtained for axial positions ranging from about 200 mm toabout 950 mm from the shoulder of the ingot are present in the graph ofFIG. 15. These results show that a pull rate profile may be determinedfor the growth of a single crystal silicon ingot such that the constantdiameter portion of the ingot may contain an axially symmetric regionhaving a width, as measured from the circumferential edge radiallytoward the central axis of the ingot, which is at least about 40% thelength of the radius of the constant diameter portion. In addition,these results show that this axially symmetric region may have a length,as measured along the central axis of the ingot, which is about 75% ofthe length of the constant diameter portion of the ingot.

Example 6

A single crystal silicon ingot have a length of about 1100 mm and adiameter of about 150 mm was grown with a decreasing pull rate. The pullrate at the shoulder of the constant diameter portion of the ingot wasabout 1 mm/min. The pull rate decreased exponentially to about 0.4mm/min., which corresponded to an axial position of about 200 mm fromthe shoulder. The pull rate then decreased linearly until a rate ofabout 0.3 mm/min. was reached near the end of the constant diameterportion of the ingot.

Under these process conditions in this particular hot zoneconfiguration, the resulting ingot contains a region wherein the axiallysymmetric region has a width which about equal to the radius of theingot. Referring now to FIGS. 16 a and 16 b, which are images producedby a scan of the minority carrier lifetime of an axial cut of a portionof the ingot following a series of oxygen precipitation heat treatments,consecutive segments of the ingot, ranging in axial position from about100 mm to about 250 mm and about 250 mm to about 400 mm are present. Itcan be seen from these figures that a region exists within the ingot,ranging in axial position from about 170 mm to about 290 mm from theshoulder, which is free of agglomerated intrinsic point defects acrossthe entire diameter. Stated another way, a region is present within theingot wherein the width of the axially symmetric region, i.e., theregion which is substantially free of agglomerated interstitial defects,is about equal to the radius of the ingot.

In addition, in a region ranging from an axially position from about 125mm to about 170 mm and from about 290 mm to greater than 400 mm thereare axially symmetric regions of interstitial dominated material free ofagglomerated intrinsic point defects surrounding a generally cylindricalcore of vacancy dominated material which is also free of agglomeratedintrinsic point defects.

Finally, in a region ranging from an axially position from about 100 mmto about 125 mm there is an axially symmetric region of interstitialdominated material free of agglomerated defects surrounding a generallycylindrical core of vacancy dominated material. Within the vacancydominated material, there is an axially symmetric region which is freeof agglomerated precipitated impurities corresponded to regions whichwere free of agglomerated interstitial defects.

Another set of the longitudinal sections was subjected to a series ofoxygen precipitation heat treatments in order to cause the nucleationand growth of new oxide clusters prior to carrier lifetime mapping.Contrast bands in lifetime mapping were utilized in order to determineand measure the shape of the instantaneous melt/solid interface atvarious axial positions in each ingot. Information on the shape of themelt/solid interface was then used, as discussed further below, toestimate the absolute value of, and the radial variation in, the averageaxial temperature gradient, G₀. This information was also used, inconjunction with the pull rate, to estimate the radial variation inv/G₀.

To more closely examine the effect growth conditions have on theresulting quality of a single crystal silicon ingot, several assumptionswere made which, based on experimental evidence available to-date, arebelieved to be justified. First, in order to simplify the treatment ofthermal history in terms of the time taken to cool to a temperature atwhich the agglomeration of interstitial defects occurs, it was assumedthat about 1050° C. is a reasonable approximation for the-temperature atwhich the agglomeration of silicon self-interstitials occurs. Thistemperature appears to coincide with changes in agglomeratedinterstitial defect density observed during experiments in whichdifferent cooling rates were employed. Although, as noted above, whetheragglomeration occurs is also a factor of the concentration ofinterstitials, it is believed that agglomeration will not occur attemperatures above about 1050° C. because, given the range ofinterstitial concentrations typical for Czochralski-type growthprocesses, it is reasonable to assume that the system will not becomecritically supersaturated with interstitials above this temperature.Stated another way, for concentrations of interstitials which aretypical for Czochralski-type growth processes, it is reasonable toassume that the system will not become critically supersaturated, andtherefore an agglomeration event will not occur, above a temperature ofabout 1050° C.

The second assumption that was made to parameterize the effect of growthconditions on the quality of single crystal silicon is that thetemperature dependence of silicon self-interstitial diffusivity isnegligible. Stated another way, it is assumed that self-interstitialsdiffuse at the same rate at all temperatures between about 1400° C. andabout 1050° C. Understanding that about 1050° C. is considered areasonable approximation for the temperature of agglomeration, theessential point of this assumption is that the details of the coolingcurve from the melting point does not matter. The diffusion distancedepends only on the total time spent cooling from the melting point toabout 1050° C.

Using the axial temperature profile data for each hot zone design andthe actual pull rate profile for a particular ingot, the total coolingtime from about 1400° C. to about 1050° C. may be calculated. It shouldbe noted that the rate at which the temperature changes for each of thehot zones was reasonably-uniform. This uniformity means that any errorin the selection of a temperature of nucleation for agglomeratedinterstitial defects, i.e. about 1050° C., will arguably lead only toscaled errors in the calculated cooling time.

In order to determine the radial extent of the vacancy dominated regionof the ingot (R_(vacancy)), or alternatively the width of the axiallysymmetric region, it was further assumed that the radius of the vacancydominated core, as determined by the lifetime map, is equivalent to thepoint at solidification where v/G₀=v/G₀ critical. Stated another way,the width of the axially symmetric region was generally assumed to bebased on the position of the V/I boundary after cooling to roomtemperature. This is pointed out because, as mentioned above, as theingot cools recombination of vacancies and silicon self-interstitialsmay occur. When recombination does occur, the actual position of the V/Iboundary shifts inwardly toward the central axis of the ingot. It isthis final position which is being referred to here.

To simplify the calculation of G₀, the average axial temperaturegradient in the crystal at the time of solidification, the melt/solidinterface shape was assumed to be the melting point isotherm. Thecrystal surface temperatures were calculated using finite elementmodeling (FEA) techniques and the details of the hot zone design. Theentire temperature field within the crystal, and therefore G₀, wasdeduced by solving Laplace's equation with the proper boundaryconditions, namely, the melting point along the melt/solid interface andthe FEA results for the surface temperature along the axis of thecrystal. The results obtained at various axial positions from one of theingots prepared and evaluated are presented in FIG. 17.

To estimate the effect that radial variations in G₀ have on the initialinterstitial concentration, a radial position R′, that is, a positionhalfway between the V/I boundary and the crystal surface, was assumed tobe the furthest point a silicon self-interstitial can be from a sink inthe ingot, whether that sink be in the vacancy dominated region or onthe crystal surface. By using the growth rate and the G₀ data for theabove ingot, the difference between the calculated v/G₀ at the positionR′ and v/G₀ at the V/I boundary (i.e., the critical v/G₀ value) providesan indication of the radial variation in the initial interstitialconcentration, as well as the effect this has on the ability for excessinterstitials to reach a sink on the crystal surface or in the vacancydominated region.

For this particular data set, it appears there is no systematicdependence of the quality of the crystal on the radial variation inv/G₀. As can be seen in FIG. 18, the axial dependence in the ingot isminimal in this sample. The growth conditions involved in this series ofexperiments represent a fairly narrow range in the radial variation ofG₀. As a result, this data set is too narrow to resolve a discernabledependence of the quality (i.e., the presence of absence of a band ofagglomerated intrinsic point defects) on the radial variation of G₀.

As noted, samples of each ingot prepared were evaluated at various axialpositions for the present or absence of agglomerated interstitialdefects. For each axial position examined, a correlation may be madebetween the quality of the sample and the width of the axially symmetricregion. Referring now to FIG. 19, a graph may be prepared which comparesthe quality of the given sample to the time the sample, at thatparticular axial position, was allowed to cool from solidification toabout 1050° C. As expected, this graph shows the width of the axiallysymmetric region (i.e., R_(crystal)−R_(vacancy)) has a strong dependenceon the cooling history of the sample within this particular temperaturerange. In order of the width of the axially symmetric region toincrease, the trend suggests that longer diffusion times, or slowercooling rates, are needed.

Based on the data present in this graph, a best fit line may becalculated which generally represents a transition in the quality of thesilicon from “good” (i.e., defect-free) to “bad” (i.e., containingdefects), as a function of the cooling time allowed for a given ingotdiameter within this particular temperature range. This generalrelationship between the width of the axially symmetric region and thecooling rate may be expressed in terms of the following equation:(R _(crystal) −R _(transition))² =D _(eff) * t _(1050° C.)wherein

-   -   R_(crystal) is the radius of the ingot,    -   R_(transition) is the radius of the axially symmetric region at        an axial position in the sample were a transition occurs in the        interstitial dominated material from being defect-free to        containing defects, or vice versa,    -   D_(eff) is a constant, about 9.3*10⁻⁴ cm²sec⁻¹, which represents        the average time and temperature of interstitial diffusivity,        and    -   t_(1050° C). is the time required for the given axial position        of the sample to cool from solidification to about 1050° C.

Referring again to FIG. 19, it can be seen that, for a given ingotdiameter, a cooling time may be estimated in order to obtain an axiallysymmetric region of a desired diameter. For example, for an ingot havinga diameter of about 150 mm, an axially symmetric region having a widthabout equal to the radius of the ingot may be obtained if, between thetemperature range of about 1410° C. and about 1050° C., this particularportion of the ingot is allowed to cool for about 10 to about 15 hours.Similarly, for an ingot having a diameter of about 200 mm, an axiallysymmetric region having a width about equal to the radius of the ingotmay be obtained if between this temperature range this particularportion of the ingot is allowed to cool for about 25 to about 35 hours.If this line is further extrapolated, cooling times of about 65 to about75 hours may be needed in order to obtain an axially symmetric regionhaving a width about equal to the radius of an ingot having a diameterof about 300 mm. It is to be noted in this regard that, as the diameterof the ingot increases, additional cooling time is required due to theincrease in distance that interstitials must diffuse in order to reachsinks at the ingot surface or the vacancy core.

Referring now to FIGS. 20, 21, 22 and 23, the effects of increasedcooling time for various ingots may be observed. Each of these figuresdepicts a portion of a ingot having a nominal diameter of 200 mm, withthe cooling time from the temperature of solidification to 1050° C.progressively increasing from FIG. 20 to FIG. 23.

Referring to FIG. 20, a portion of an ingot, ranging in axial positionfrom about 235 mm to about 350 mm from the shoulder, is shown. At anaxial position of about 255 mm, the width of the axially symmetricregion free of agglomerated interstitial defects is at a maximum, whichis about 45% of the radius of the ingot. Beyond this position, atransition occurs from a region which is free of such defects, to aregion in which such defects are present.

Referring now to FIG. 21, a portion of an ingot, ranging in axialposition from about 305 mm to about 460 mm from the shoulder, is shown.At an axial position of about 360 mm, the width of the axially symmetricregion free of agglomerated interstitial defects is at a maximum, whichis about 65% of the radius of the ingot. Beyond this position, defectformation begins.

Referring now to FIG. 22, a portion of an ingot, ranging in axialposition from about 140 mm to about 275 mm from the shoulder, is shown.At an axial position of about 210 mm, the width of the axially symmetricregion is about equal to the radius of the ingot; that is, a smallportion of the ingot within this range is free of agglomerated intrinsicpoint defects.

Referring now to FIG. 23, a portion of an ingot, ranging in axialposition from about 600 mm to about 730 mm from the shoulder, is shown.Over an axial position ranging from about 640 mm to about 665 mm, thewidth of the axially symmetric region is about equal to the radius ofthe ingot. In addition, the length of the ingot segment in which thewidth of the axially symmetric region is about equal to the radius ofthe ingot is greater than what is observed in connection with the ingotof FIG. 22.

When viewed in combination, therefore, FIGS. 20, 21, 22, and 23demonstrate the effect of cooling time to 1050° C. upon the width andthe length of the defect-free, axially symmetric region. In general, theregions containing agglomerated interstitial defects occurred as aresult of a continued decrease of the crystal pull rate leading to aninitial interstitial concentration which was too large to reduce for thecooling time of that portion of the crystal. A greater length of theaxially symmetric region means a larger range of pull rates (i.e.,initial interstitial concentration) are available for the growth of suchdefect-free material. Increasing the cooling time allows for initiallyhigher concentration of interstitials, as sufficient time for radialdiffusion may be achieved to suppress the concentration below thecritical concentration required for agglomeration of interstitialdefects. Stated in other words, for longer cooling times, somewhat lowerpull rates (and, therefore, higher initial interstitial concentrations)will still lead to maximum axially symmetric region 6. Therefore, longercooling times lead to an increase in the allowable pull rate variationabout the condition required for maximum axially symmetric regiondiameter and ease the restrictions on process control. As a result, theprocess for an axially symmetric region over large lengths of the ingotbecomes easier.

Referring again to FIG. 23, over an axial position ranging from about665 mm to greater than 730 mm from the shoulder of crystal, a region ofvacancy dominated material free of agglomerated defects is present inwhich the width of the region is equal to the radius of the ingot.

As can be seen from the above data, by means of controlling the coolingrate, the concentration of self-interstitials may be suppressed byallowing more time for interstitials to diffuse to regions where theymay be annihilated. As a result, the formation of agglomeratedinterstitial defects is prevented within significant portion of thesingle crystal silicon ingot.

In view of the above, it will be seen that the several objects of theinvention are achieved.

As various changes could be made in the above compositions and processeswithout departing from the scope of the invention, it is intended thatall matter contained in the above description be interpreted asillustrative and not in a limiting sense.

1. A process for growing a single crystal silicon ingot in which theingot comprises a central axis, a seed-cone, an end-cone and a constantdiameter portion between the seed-cone and the end-cone having acircumferential edge and a radius extending from the central axis to thecircumferential edge, the ingot being grown from a silicon melt and thencooled from the solidification temperature in accordance with theCzochralski method, the process comprising controlling a growthvelocity, v, and an average axial temperature gradient, G₀, during thegrowth of the constant diameter portion of the crystal over thetemperature range from solidification to a temperature of no less thanabout 1325° C., to cause the formation of a first axially symmetricalregion in which vacancies, upon cooling of the ingot from thesolidification temperature, are the predominant intrinsic point defectand which is substantially free of agglomerated intrinsic point defectswherein the first axially symmetric region has a width of at least about50% of the radius of the constant diameter portion of the ingot.
 2. Theprocess of claim 1 wherein the first axially symmetric region has alength which is at least about 40% the length of the constant diameterportion of the ingot.
 3. The process of claim 2 wherein the ingotcomprises a second axially symmetric region which is concentric withsaid first axially symmetric region, the second axially symmetric regioncontaining self-interstitial atoms as the predominant intrinsic pointdefect and being substantially free of agglomerated siliconself-interstitial intrinsic point defects.
 4. The process of claim 1wherein the first axially symmetric region has a length which is atleast about 80% the length of the constant diameter portion of theingot.
 5. The process of claim 4 wherein the ingot comprises a secondaxially symmetric region which is concentric with said first axiallysymmetric region, the second axially symmetric region containingself-interstitial atoms as the predominant intrinsic point defect andbeing substantially free of agglomerated silicon self-interstitialintrinsic point defects.
 6. The process of claim 1 wherein the firstaxially symmetric region has a width which is at least about 80% thelength of the radius of the constant diameter portion of the ingot. 7.The process of claim 6 wherein the first axially symmetric region has alength which is at least about 40% the length of the constant diameterportion of the ingot.
 8. The process of claim 7 wherein the ingotcomprises a second axially symmetric region which is concentric withsaid first axially symmetric region, the second axially symmetric regioncontaining self-interstitial atoms as the predominant intrinsic pointdefect and being substantially free of agglomerated siliconself-interstitial intrinsic point defects.
 9. The process of claim 6wherein the first axially symmetric region has a length which is atleast about 80% the length of the constant diameter portion of theingot.
 10. The process of claim 9 wherein the ingot comprises a secondaxially symmetric region which is concentric with said first axiallysymmetric region, the second axially symmetric region containingself-interstitial atoms as the predominant intrinsic point defect andbeing substantially free of agglomerated silicon self-interstitialintrinsic point defects.
 11. The process of claim 1 wherein the firstaxially symmetric region has a width which is about equal to the radiusof the constant diameter portion of the ingot.
 12. The process of claim11 wherein the first axially symmetric region has a length which is atleast about 20% the length of the constant diameter portion of theingot.
 13. The process of claim 11 wherein the first axially symmetricregion has a length which is at least about 40% the length of theconstant diameter portion of the ingot.